The goal of this project is to identify phenotypic alterations due to immunosenescence, which can affect the development of endotoxin tolerance in the aged host. Sepsis is a clinically defined, potentially devastating derangement of homeostatic functions, usually triggered by exposure to bacterial products like endotoxin. It causes a high degree of morbidity and mortality in elderly patients. The innate immune system is protective against endotoxins, but inappropriate activation of immune cells can lead to a systemic inflammatory response and sepsis. Immune responses to endotoxin can be suppressed by the development of endotoxin tolerance in a host repeatedly exposed to low dose endotoxin. The role of endotoxin tolerance in the adverse outcomes of abdominal sepsis often observed in the elderly has not been studied. Our preliminary data demonstrates that peripheral blood mononuclear cells (PBMCs) from aged animals differ in their phenotypic characteristics from PBMCs from young animals when they are pre- conditioned with low dose endotoxin prior to stimulation with high dose endotoxin. These include cell surface expression of Toll like receptors and mRNA and protein expression of downstream signaling molecules such as IRAK-1 and IRAK-M. Based on these observations, our hypothesis is that alterations in cell surface expression of Toll like receptors, altered IRAK-1 and IRAK-M activity, and altered NF-?B activation can interfere with development of endotoxin tolerance in the aged animal. We propose to pre-condition young, mature and aged Brown Norway rats with intra-peritoneal (i.p.) injection of low dose Lipopolysaccharide (LPS) for three consecutive days. In these pre-conditioned animals, our specific aims are: (1) to determine ex vivo the development of the tolerance phenotype in PBMCs peritoneal cells harvested from young, mature and aged rats and (2) to determine in vivo if young, mature and aged rats differ in their survival characteristics and in their response to i.p. injection of high dose LPS or cecal ligation and puncture. These studies will help determine the molecular basis of endotoxin tolerance, and devise rational strategies in the use of biological response modifiers to treat the elderly patient with sepsis, thus improving the standard of geriatric care. We propose to investigate whether the phenotypic alterations due to immunosenescence lead to defective endotoxin tolerance in a rodent model of aging. Little is known about the role of endotoxin tolerance in the adverse outcomes of abdominal sepsis often observed in the elderly. Our studies will help determine the molecular basis of endotoxin tolerance, and devise rational strategies in the use of biological response modifiers to treat the elderly patient with sepsis, thus improving the standard of geriatric care. [unreadable] [unreadable] [unreadable]